1. Field of the Invention
The present invention relates to zoom lens systems, and more particularly to wide-angle, compact inner-focus zoom lens systems.
2. Description of Related Art
Allowing the changing of focal length without interchanging lenses, zoom lens systems are advantageous in terms of convenience and cost. Quite often, however, they fail to offer sufficient imaging performance over the entire range of focal length or of conjugate length, and, at close distances in particular, it is difficult to obtain high imaging performance at all. Also difficult is to suppress variation in view angle during focusing from the infinite distance to a close distance, and few techniques to that end have been known. As a solution, a technique called floating involving the use of a plurality of lens groups as focusing lens groups are proposed in Patent Documents 1 to 5 listed below.
For example, Patent Document 1 discloses a zoom lens system of a negative-leading type and that adopts inner focusing in the second lens group. Patent Document 2 discloses a zoom lens system of a negative-leading type and that adopts focusing a negative and a positive lens group provided in the first lens group. Patent Document 3 discloses a zoom lens system of a negative-leading type and that adopts focusing two negative lens groups provided in the first lens group. Patent Document 4 discloses a zoom lens system of a negative-leading type and that adopts focusing a negative and a positive lens group provided in the first lens group. Patent Document 5 discloses a zoom lens system of a positive-leading type and that includes a first lens group composed of a stationary negative lens group and two positive lens groups, wherein focusing is achieved by moving the two positive lens groups in opposite directions.
Patent Document 1: JP-A-2009-169051
Patent Document 2: JP-A-H11-095100
Patent Document 3: JP-A-2002-357771
Patent Document 4: JP-A-2008-257005
Patent Document 5: JP-A-H09-258102
Inconveniently, in the zoom lens system disclosed in Patent Document 1, a focusing lens group is shared as a zooming lens group, and this results in a large variation in focal length (in view angle) during focusing. In the zoom lens systems disclosed in Patent Documents 2 to 4, all the lens groups within the first lens group are moved, with the natural result that a lens group with a high optical power needs to be moved for focusing; this makes it impossible to suppress variation in view angle. Moreover, in the zoom lens systems disclosed in Patent Documents 1 to 4, the front lens element is movable, and accordingly, to secure a sufficient amount of peripheral light over the entire range of conjugate length, the front lens diameter needs to allow for a margin. In the zoom lens system disclosed in Patent Document 5, the first lens group has a positive optical power, and thus aiming at a wider angle leads to a larger lens diameter.